Silicones are polymeric materials that have one characteristic in common: the polymer backbone is made of an alternate succession of Si and O atoms, joined together via strong, covalent inter-atomic bonds. The Si atoms are coupled to two adjacent O atoms and two organic radicals, i.e., C—H or C—R, where R is an organic group or moiety. The various silicones only differ from each other via these organic radicals, e.g. methyl (—CH3), vinyl (—HC═CH2), or other organic functional group. Silicones are variously referred to as “polymerized siloxanes,” “polysiloxanes,” and “silicone polymers.” “Silicone rubbers” are included in this definition but typically include one or more additives, such as fillers, plasticizers, and crosslinkers. We use the term “silicone” in its broader sense to refer to silicone polymers, whether or not modified with one or more additional components.
Silicones are notably neutral to the environment, asserting in particular no chemical interaction with foreign molecules. They also exhibit very low electrical conductivity and are fully transparent to visible or infra-red light. They absorb light photons in the UV range, typically at and below 280 nm wavelength (i.e. at and above 4.4 eV photon energy).
In order to allow chemical coupling of silicone materials to foreign species, it is necessary to “open” their structure, i.e. to modify irreversibly their atom assembly via breaking irreversibly some of the inter-atomic bonds. Unfortunately, this may not be accessible to mechanical action. In effect, given that these materials are elastic, they may change considerably their configuration through pulling without breaking bonds, this being in particular a consequence of the rotational symmetry of the Si—O bonds. Similarly, opening the silicone structure may not be feasible via thermal means. Silicone does not melt, sublimate or evaporate but rather condenses and transforms in a glassy and extremely fragile network at temperatures exceeding 230° C.
Due to their chemical inertness, silicones are recognized as biocompatible materials and are widely used in practical medical implants. For example, an epiretinal visual prosthesis (a microelectrode array (MEA) imbedded into or onto a silicone substrate, or applied using photolithography) is a device that can be implanted on the retina and converts images into electrical signals that stimulate the retina. The images are received from an external camera and transfer the visual information to the MEA.
Unlike cells, which attach to their extracellular environment via integral membrane proteins called integrins, MEAs and other medical implants are generally affixed to adjacent tissue using surgical tacks or adhesives, which may be actually or potentially harmful to the tissue and, therefore, may limit the actual lifetime of the implant function. For example, a method currently used to fix an epiretinal visual prosthesis in place utilizes surgical tacks secured to the retina, which cause local pressure effects, local tissue destruction, and vascular leakage. Pressure is a crucial component of the cellular environment and can lead to pathology if it varies beyond the normal range. Disorders of this relationship can lead to disease states, such as glaucoma, in which retinal ganglion cells undergo apoptosis and necrosis.
A major obstacle faced by bioengineers has been the ability to attach proteins to biocompatible substrates, and there is a continuing need for biocompatible materials and less destructive methods of attaching them to tissues. If silicone implants are to be fixed in place in the body, a way must be found to “activate” the silicone polymers to permit them to bond more readily to one or more compounds, such as cellular or extracellular proteins.